Re: `@need` badly influences top margin

[Gavin Smith]

On Thu, Aug 18, 2022 at 11:49:56AM +0000, Werner LEMBERG wrote:
> 
> [texinfo.tex version 2022-08-13.17]
> 
> 
> Look at the two attached documents, with and without `@need`.  If
> `@need` is used, the `@section foo` title starting a new page is moved
> down a little bit for no reason.

I managed to reproduce the extra space after @need at the top of the
page.

I found with @need, the section skip was being output.  This showed up
with @tracingboxes as

..@glue 12.0 plus 3.0 minus 2.0

which is the same as \secheadingskip, but this was not present without @need.

When the \vtop appears at the beginning of the page, glue and penalties
are not discarded, as the \vtop is not a "discardable item".

One possible fix is to replace the \vtop with vertical glue:

diff --git a/doc/texinfo.tex b/doc/texinfo.tex
index c7ebff708f..29f2fbc3bf 100644
--- a/doc/texinfo.tex
+++ b/doc/texinfo.tex
@@ -729,7 +729,8 @@ where each line of input produces a line of output.}
     % Do a \strut just to make the height of this box be normal, so the
     % normal leading is inserted relative to the preceding line.
     % And a page break here is fine.
-    \vtop to #1\mil{\strut\vfil}%
+    %\vtop to #1\mil{\strut\vfil}%
+    \vskip #1\mil
     %
     % TeX does not even consider page breaks if a penalty added to the
     % main vertical list is 10000 or more.  But in order to see if the

This is one of the ways that the LaTeX needspace package does it
(https://ctan.org/pkg/needspace?lang=en).

However, more testing is required.  If you're aware of places where
@need makes a difference, it would help if you could check them with
this change.

I don't completely understand what is going on with \strut here either.